Disorders of attention appear to be involved in many clinical disorders, not only those in which attentional and cognitive dysfunction label the disorder, as in Attention Deficit Disorder, learning disabilities, and mental retardation, but also in disorders, such as schizophrenia and psychopathy, where the relationship is less obvious. The proposed research aims to understand, in normal young adults, how two automatic processes function in determining when or whether attention is directed to stimulus changes in the environment. One process is the inhibition of the startle blink reflex by any kind of brief change in stimulation occurring within a few hundred ms of a startle-eliciting stimulus. This process requires only midbrain structures and is found in many species, yet does not mature until adolescence. It has been described as a gating mechanism but how it functions is not well understood. This research aims to determine the conditions critical for its elicitation and how it affects voluntary detection and discrimination of stimulus inputs. The second automatic process is the non-signal orienting response (OR) to novel or unexpected changes in stimulation. This is often referred to as passive attention. Although many of its characteristics are mature in young infants, a reputed component seen in scalp-recorded potentials from frontal electrodes is immature until late adolescence. The research goal, in this case, is to identify characteristics of scalp-recorded brain potentials which reflect identification of stimuli mismatching what has been presented in the recent past, the conditions under which this leads to a shift of attention, and the consequences such a shift has on subsequent stimulation. In studying both processes, both behavioral and psychophysiological measurements will be made, including recording of cardiac rate changes and electromyography from m. orbicularis oculi, in addition to scalp potentials.